The present invention relates to a cordless key telephone system.
Cordless key telephone systems have been in widespread use replacing wired key telephone systems to reduce the amount of cables and wires laid on office floors. The system usually includes a main controller which is coupled to the public or private switched telephone network via exchange lines to receive incoming calls and originate outgoing calls. The floor space is divided into several service zones to which control channels are uniquely assigned. One or more access units are located in each service zone, and are coupled to the main controller to relay control signals between it and the cordless stations before establishing a connection through a speech channel. The cordless station is provided with a field intensity detector for alerting the user when the field intensity drops below an acceptable level. When the user of a given cordless station is in conversation with a distant network-side party via an exchange line while walking across a boundary between adjacent zones, the user will be alerted by the field intensity detector and would request the distant party to wait for a moment, and depress a "hold" key to trigger a signal. The main controller receives this signal to hold the exchange line and sends back a signal to the cordless station to give a visual indication to the user, indicating that the exchange line is kept in a line-hold mode, while triggering a timeout circuit to measure the length of time in which the line-hold mode is continued. If a prescribed period expires the exchange line is forcibly disconnected. Upon seeing the visual indication, the user is supposed to depress an end-of-call key to allow the cordless station to switch to standby mode. The end-of-call key causes a signal to be transmitted to the main controller to record the status of the call in a call status memory, so that the exchange line can be accessed exclusively from the cordless station. On entering the second zone, the station user is supposed to depress a call request key signalling the origination of a call to an access unit of the second zone, and thence to the main controller. On receiving the call request, the access unit searches speech channels to assign an idle channel and returns a channel assignment signal to the cordless station to cause it to switch to the assigned channel. On the other hand, the main controller responds to the call request by accessing the call status memory and establishing a connection between the exchange line and the access unit of the second zone according to the recorded call status.
Since the conventional cordless key telephone system processes zone switching events in response to manual key commands, delayed operations of the keys may inadvertently allow the timeout circuit to disconnect the exchange line, leaving the network-side party in a state of embarrassment.